The neuronal circuitryof the cerebellum appears to be relativelysimple and well defined, but the computation performed by this circuitry is not understood. In addition, forms of synaptic plasticityhave been defined in the cerebellum that may be responsiblefor motor learning, but criticalquestions regarding this plasticityare still unanswered. The objective of the proposed research is to address fundamental issues cerebellar circuitry and plasticitythrough a study of the unique cerebellum of mormyridelectricfish. Two features of the mormyrid cerebellum make itwell suitedto address these issues: 1) Purkinjecells and cerebellar output cells (cells of the cerebellar nuclei in mammals) are adjacent to each other in mormyridfish, making possibleexperiments on integrationand plasticityat the important synapse between these two cell types that can not be done in mammals. 2) The regionsof terminationfor climbingfiber and parallel fiber inputs on Purkinjecellsare distinctand well separated in the mormyrid but not inthe mammal. This separation facilitates the determination of intrinsicdifferences between the two dendritic regions and the determination of how the two inputs interactto generate synaptic plasticity. The project has two Specific Aims: 1) to determine the basic synaptic and intrinsicphysiologyof Purkinjeand cerebellar output cells in mormyrid electricfish; and 2) to determine the types of plasticityat parallel fiber and Purkinje cellsynapses in mormyrid electricfish. The primary method will be whole-cell patch recordingin in vitro slices, combined with labelingof recorded elements for morphologicalidentification.